The invention relates to fermented products based on plant raw materials.
It is known to use plant raw materials, in particular cereals, as substitutes or supplements for raw materials of milk origin in the manufacture of fermented food products, with the aim of combining the beneficial properties of these plant products with those of lactic ferments.
For example, application EP 0,622,024 describes a product intended for early childhood feeding, consisting of 95% of yoghurt and 5% of a cereals meal mixture (oats, rice and wheat) and of soy extract, pasteurized beforehand and then fermented with Bifidobacterium infantis and optionally Streptococcus thermophilus, to a pH of about 4.6 to 4.7. It is indicated that this fermented supplement improves the nutritional value and the digestibility of the yoghurt, while providing nutrients which are essential for the growth of young children in a directly assimilable form, and that, in addition, the presence of the soy extract promotes the fermentative activity of Bifidobacterium infantis, and improves its viability. However, this document gives no precise information on the storage life of the fermented supplement or on that of the finished product (supplement+yoghurt), or on the effective rate of survival of the bacteria in the fermented supplement or in the final product.
Application EP 0,568,530 describes a product prepared from oat bran, mixed with water and gelatinized by a heat treatment at 100xc2x0 C. for 20 min, and then fermented with lactic acid bacteria. The final product obtained is rich in fibre (xcex2-glucans), obtained from the oat bran, and contains live lactobacilli (105 to 108 CFU/g). This document also indicates that the product obtained can be stored for more than 2 weeks; the lactic acid content increases during storage.
Application EP 0,415,941 describes a liquid product intended for enteral feeding by tube, or for the preparation of a health drink. This product is prepared from oat flour hydrolysed with an xcex1-amylase and a xcex2-glucanase, optionally supplemented with soy flour in order to supplement it with proteins and fats, and then fermented by lactobacilli, in particular Lactobacillus reuterii. The liquid product obtained contains viable lactobacilli, and preserves its properties for about 8 days.
The main problem posed in the manufacture of this type of product is to reconcile, during the manufacture, packaging and storage, the longest possible survival of the lactic ferments, and the preservation of the organoleptic qualities of the product. For example, the post-acidification described in application EP 0,568,530, which reflects the viability of the lactobacilli, can constitute a disadvantage insofar as it causes a modification of the taste of the product, which modification is not necessarily liked by consumers.
The aim of the present invention is the production of food products based on raw materials of plant origin fermented by lactic acid bacteria, which are capable of preserving, after packaging and storage, both a high level of viability of the lactic ferments, and intact organoleptic qualities.
The inventors have observed that some lactic ferments, such as Streptococcus thermophilus, exhibit, when they are used to ferment raw materials of plant origin, a reduced post-acidification while retaining sufficient growth and viability. These ferments are therefore particularly suitable for the production of food products possessing the above-defined properties. The inventors have also observed that the use, as raw material of plant origin, of a mixture based on soy milk and a cereal hydrolysate by at least one amylase, and/or almond milk, provided a substrate which is particularly favourable for good growth of these ferments, and made it possible to carry out the fermentation under optimum conditions.
The subject of the present invention is a method for producing a food product by lactic acid fermentation of raw materials of plant origin, which method is characterized in that it comprises:
the preparation of a mixture comprising:
a) soy milk, and
b) a hydrolysate of at least one cereal by at least one amylase, and/or almond milk,
the fermentation of the mixture with a lactic ferment comprising at least one strain of Streptococcus thermophilus. 
Soy milk which can be used for carrying out the method in accordance with the present invention is for example the preparation known by the name of xe2x80x9ctonyuxe2x80x9d obtained from whole soy beans, and which is the subject of an AFNOR standard (NF V 29-001). Briefly, to obtain the tonyu, the soy beans are shelled and then mixed with water and ground in the hot state ( greater than 85xc2x0 C.). The ground product is separated after settling out so as to separate the solid residue, called xe2x80x9cokaraxe2x80x9d, from the soy milk which constitutes the tonyu. The tonyu is then sterilized and then cooled to 40xc2x0 C.
The cereal hydrolysate used to carry out the method in accordance with the present invention is obtained, for example, from a flour or from cereal flakes, which are hydrolysed, in a manner known per se, with at least one amylase, preferably with a xcex2-amylase and an xcex1-amylase. If it is desired to obtain a final product rich in fibres, it is possible to use a hydrolysate obtained from whole cereal flakes or flour. It is possible to advantageously use a rice, barley, wheat or oat hydrolysate, or a mixture thereof. In particular, the use of an oat hydrolysate makes it possible to obtain a product with a higher xcex2-glucan content. By way of example, an oat hydrolysate which can be used in the context of the present invention is described in application PCT WO 95/07628.
For the sake of convenience, the cereal hydrolysate may be provided in powdered form; it is possible, in this case, to either add it directly to the soy milk, or to reconstitute, prior to the mixing with the soy milk, a liquid hydrolysate (also called hereinafter xe2x80x9chydrolysed cereal juicexe2x80x9d), preferably comprising between 5 and 20% by weight of dry matter, by the addition of an appropriate quantity of water.
All or some of the hydrolysed cereal juice may be replaced with almond milk.
The use of the soy milk/constituent b) mixture ensures excellent fermentation conditions which allow the formation of a regular network between the molecules of the various constituents of the product, and confer on it a smooth and homogeneous texture.
Advantageously, to obtain a final product which is optimal both from the point of view of the organoleptic qualities and of the texture, the soy milk/constituent b) mixture comprises between 2.5% and 9%, and preferably about 5% (by weight) of dry matter obtained from the soy milk. The latter proportions give a raw material having contents of macronutrients (proteins, lipids and carbohydrates) which are equivalent to those of semi-skimmed milk.
If desired, it is possible to add to the mixture texturing agents, among those conventionally used in the manufacture of food products, in particular fermented dairy products. For example, starch may be used, in proportions of 2 to 3% by weight.
It is also possible to add one or more antioxidants, for example vitamin E, in order to prevent the oxidation of the polyunsaturated fatty acids present in the raw materials. If it is desired to increase the fibre content of the product, it is also possible to add, at this stage, dietary fibres, for example fructooligosaccharides (FOS).
After incorporation of the various constituents, the mixture is kept stirring (in general for 30 min to 1 hour), in order to rehydrate the components provided in solid form, and then sterilized (for example 130xc2x0 C. for 2 to 20 s). It is possible to replace the sterilization step by a pasteurization heat treatment (for example at 95xc2x0 C., for 5 min to 10 min) or by ultrahigh-temperature sterilization by direct injection of steam into the product (for example at 135xc2x0 C., for 3 s).
Prior to the inoculation with the lactic ferments, the mixture is cooled to a temperature of about 25xc2x0 C. to 45xc2x0 C.
It is then inoculated with an inoculum of lactic acid bacteria, at an initial bacterial concentration of at least 104 to 105 CFU/g in the fermentation medium.
The inventors have, in addition, observed that some strains of Streptococcus thermophilus, while inducing only a very low post-acidification, retained an excellent viability. These strains, called xe2x80x9creduced post-acidification strainsxe2x80x9d are most particularly suitable for carrying out the present invention; they may be selected on the basis of their viability and post-acidification characteristics, after culturing on a 50/50 (v/v) mixture of the a) and b) constituents defined above, and storage and at a temperature of about 10xc2x0 C.
Advantageously, a strain will be chosen which exhibits, under these conditions, the following characteristics:
the bacterial population, after 28 days, (for an initial inoculation of about 104 to 105 CFU/g), is at least 106 CFU/g, preferably between 107 and 109 CFU/g;
the difference between the curd settling pH and the pH after 28 days (xcex94pH D0-D28) is less than or equal to 0.2.
A strain of Streptococcus thermophilus which can be used for carrying out the method in accordance with the invention is for example the strain deposited, according to the Budapest Treaty, on Dec. 30, 1994, at the CNCM (Collection Nationale de Cultures de Microorganismes) held by Institut Pasteur, 25 rue du Docteur Roux, in Paris, under the number I-1520.
For carrying out the method in accordance with the invention, it is also possible to combine the strain(s) of Streptococcus thermophilus with at least one lactic acid bacterium strain chosen from the group consisting of Bifidobacterium ssp., Lactobacillus ssp. (in particular L. plantarum, L. pentosus, L. bulgaricus, L. casei, L. acidophilus), Lactococcus ssp., Pediococcus ssp. and the like. The combined strain or strains are chosen according to the taste which it is desired to give to the final product. Depending on the combined strain, it is possible to observe a higher post-acidification than that observed with Streptococcus thermophilus alone; nevertheless, the xcex94pH D0-D28 remains less than or equal to 0.6.
Depending on the ferment used, the fermentation is carried out for 3 to 24 hours, and stopped at a pH of between 4 and 5.
At the end of the fermentation, the product may be made into a smooth paste in order to give it the desired texture. For example, if it is desired to obtain a drink, the making into a smooth paste is enhanced to increase the shearing and obtain a fluid texture. Stabilizing agents may also be added.
The product is then cooled to 20xc2x0 C. It is possible, if desired, to add sugar, for example in the form of a sugar syrup (sucrose, fructose, cane sugar, honey), flavourings, fruits (preparation of fruits, concentrate, compote, juice, syrup), prior to the packaging and to the cooling to the storage temperature. The product can be stored at a temperature preferably of between 4xc2x0 C. and 15xc2x0 C.
The subject of the present invention is also the fermented products based on plant raw materials which can be obtained by the method in accordance with the invention, as defined above.
For example, products in accordance with the invention, obtained by fermentation with one or more strains of Streptococcus thermophilus with reduced post-acidification, optionally combined with Bifidobacterium, exhibit the following characteristics, after 28 days of storage at a temperature of about 10xc2x0 C.:
they contain at least 106, preferably between 107 and 109 CFU/ml of Streptococcus thermophilus; 
the difference between their pH at the end of fermentation and their pH after 28 days of storage at a temperature of about 10xc2x0 C. is less than 0.2 pH units.
It therefore appears that the lactic ferments present in the products obtained in accordance with the invention retain a high rate of survival, without this resulting in a reduction in the organoleptic qualities of the product, which would result in particular from an excessive post-acidification.
These characteristics allow in particular easy industrialization insofar as the finished product withstands buffer storage and long packaging times, without degradation of its qualities.
The invention will be understood more clearly with the aid of the additional description which follows, which refers to examples illustrating the preparation and the properties of the fermented products in accordance with the invention.
It should be clearly understood, however, that these examples are given solely by way of illustration of the subject of the invention and do not constitute in any manner a limitation thereto.